Doctor blade devices operating with magnetically generated pressing forces are known. Doctor blade rods which are either made entirely of magnetizable material or are made of several parts of which at least one is magnetizable are also known, either as components of doctor blade devices or, in some special cases, as a magnetizable cylindrical bar (magnetic roll wiper) operating by itself as a doctor blade device.
Further a spreading blade device is known, which is constructed as a spreading blade profiled rod or as a twin spreading blade and consists of two such spreading blade profiled rods supported at its ends in a roller guide, i.e. each in a respective guide roller, by means of axle pivots.
In the case of the magnetic roll wiper, the available magnetic force acts primarily to press the wiper against a substrate and is, as a secondary function, a location-determining force. This location-determining action derives from the magnetically performed main function resulting from the pressure exerted upon the doctor device, without reducing thereby the pressure force in any way.
In the case of the spreading blade, or the twin spreading blade profiled rod, which laterally is roller-supported or roller-guided, the situation is different. The pressure exerted upon the doctor blades is weakened by two factors: namely of which the first factor the weight of the doctor blade profiled rod supported to a large extent by lateral roller guides and because the magnetic force generated by the magnetic device in the magnetizable part of the spreading blade acts only partially as a pressure force. A fraction of this latter force acts just like the inherent weight through the axial pivots upon the lateral roller guides, and is not effective to the application process as pressure force, i.e. as the actually working force.
A laterally roller-guided doctor blade device like the one described above is just as much exposed to the danger of bending as any other doctor blade device held or fastened at its ends. As is known, with increasing work width or length of the profiled rods or increasing magnetic pressing force, this bending danger increases with the square of the application width, or the length of the doctor blade device.
As opposed to the magnetically pressed doctor blade devices, magnetically pressed spreading blade devices of the aforementioned kind have not proven to be useful in practice.
A drawback of the blades described above is the fact that the amount of substance to be applied is determined by the diameter of the wiper roller and therefore changes in the output of substance to be applied can be brought about only by modifying the diameter of the roller, i.e. by replacing one wiper roller with a diameter A with another wiper roller of a diameter B, which can be done only by interrupting the operation.
The disadvantage described above has been partially overcome with doctor roller devices having a profiled rod fastened to the doctor blade device upstream of the wiper roller considered in the direction of movement, this profiled rod being movable per se as an element or being movable via the doctor blade device, i.e. its local position with respect to the application zone is changeable. With auxiliary or additional devices of this kind it is possible within certain limits to perform a certain quantitative control also during operation, without having to interrupt production.